1. Field of the Invention
The present invention relates to a display panel and a method of manufacturing the same. For example, the present invention relates to a display panel having contact holes and a method of manufacturing the same.
2. Description of Related Art
Examples of display panels include a simple matrix panel and an active matrix panel having thin film transistors (TFTs) serving as switching elements. In recent years, an active matrix display device using TFTs has been widely put to practical use, because of its visual quality. Examples of such a display panel include a liquid crystal display panel and an electro-luminescence (EL) display panel. The liquid crystal display panel serving as an electro-optic element using a liquid crystal has been vigorously applied to a display device. The liquid crystal display panel with characteristics of being thin and low power consumption is widely used for office automation equipment such as a personal computer, a cellular phone, a portable information device such as a personal digital assistance, a car navigation system with a liquid crystal display monitor, a camera-integrated VTR, or a television. In recent years, there has been also employed an electro-luminescence (EL) display panel characterized by having a high level of visibility, a high-speed response, a high view angle, a thin shape, a light weight, and the like provided through light-emitting display.
A liquid crystal display panel mounted on a liquid crystal display device itself does not emit light, unlike a display using a conventional cathode-ray tube or an electro-luminescence (EL) device. For this reason, there is often employed a transmissive liquid crystal display panel having a illuminating device which includes a fluorescent tube called a backlight and which is installed on a rear surface or on a side surface of the liquid crystal display panel. In the transmissive liquid crystal display panel, light is illuminated by the backlight and an amount of transmission of light is controlled, to thereby display an image. The transmissive liquid crystal display panel has a high level of visibility in a dark place, but has a low level of visibility in a bright place.
For this reason, for a personal digital assistance and the like, which are often used to be carried outdoors on a daily basis, there is used a reflective liquid crystal display panel utilizing surrounding light instead of the backlight as a light source. The reflective liquid crystal display panel includes a reflective film, instead of a transparent film, used for a pixel electrode portion of a substrate. The surrounding light is reflected on a surface of the reflective film, whereby an image is displayed. Thus, the reflective liquid crystal display panel requires no backlight, which is advantageous in that a power consumption can be reduced. However, the reflective liquid crystal display panel also has a disadvantage in that the visibility is extremely lowered when the surrounding light is not sufficient.
In order to solve the above-mentioned problems, there is employed a transflective liquid crystal display panel which allows a part of light from the backlight to be transmitted and which allows a part of surrounding light to be reflected (see FIGS. 1 to 4 of Japanese Unexamined Patent Application Publication No. 11-281992). The transflective liquid crystal display panel includes a transmission portion having a transparent film used for a pixel electrode portion, and a reflection portion having a reflective film used for a pixel electrode portion. Thus, transmissive display as well as reflective display can be realized using a single liquid crystal display panel.
As described above, there are conventionally employed three main types of liquid crystal display panels, that is, transmissive, reflective, and transflective liquid crystal display panels. In this case, the typical configurations of those liquid crystal display panels are shown in FIGS. 8 to 13. First, referring to FIGS. 8 and 9, a description is given of the configuration of the transmissive liquid crystal display panel. FIG. 8 is a plan view showing the configuration of each of a TFT and a pixel electrode portion of the transmissive liquid crystal display panel. FIG. 9 is a cross-sectional view showing the configuration of each of the TFT and the pixel electrode portion of the transmissive liquid crystal display panel. Note that FIG. 9 also shows the configuration of the cross section of each of a gate terminal portion and a source terminal portion. Those components each serve as a connection portion to be connected with a drive circuit so as to transmit a scan signal and a video signal (display signal) from the drive circuit.
The transmissive liquid crystal display panel includes a TFT 100 provided for each pixel. By use of a transmissive electrode 101 as a pixel electrode, the transmissive electrode 101 is formed on substantially the entire surface of each pixel. In general, as the transmissive electrode 101, there is used a transparent conductive film such as ITO made of indium oxide or tin oxide. On a gate terminal 102 and a source terminal 103, each of which serves as a connection portion to be connected with the drive circuit, a gate terminal pad 104 and a source terminal pad 105 are formed, respectively. The TFT 100 and the transmissive electrode 101, the gate terminal 102 and the gate terminal pad 104, and the source terminal 103 and the source terminal pad 105 are connected to each other via contact holes 106.
Next, referring to FIGS. 10 and 11, a description is given of the configuration of the transflective liquid crystal display panel. FIG. 10 is a plan view showing the configuration of each of a TFT and a pixel electrode portion of the transflective liquid crystal display panel. FIG. 11 is a cross-sectional view showing the configuration of each of the TFT and the pixel electrode portion of the transflective liquid crystal display panel. Note that FIG. 11 also shows the configuration of the cross section of each of a gate terminal portion and a source terminal portion.
As in the case of the transmissive liquid crystal display panel, the transflective liquid crystal display panel includes the TFT 100, the transmissive electrode 101, the gate terminal 102, the source terminal 103, the gate terminal pad 104, and the source terminal pad 105. In a similar manner as in the transmissive liquid crystal display panel, those components are connected to each other via the contact holes 106. The transflective liquid crystal display panel has a reflective electrode 107 formed in an area corresponding to substantially a half of a pixel on a side of the TFT 100. Thus, as the pixel electrodes, the transmissive electrode 101 and the reflective electrode 107 are used. As the reflective electrode 107, Ag or Al is used in many cases.
Next, referring to FIGS. 12 and 13, a description is given of the configuration of the reflective liquid crystal display panel. FIG. 12 is a plan view showing the configuration of each of a TFT and a pixel electrode portion of the reflective liquid crystal display panel. FIG. 13 is a cross-sectional view showing the configuration of each of the TFT and the pixel electrode portion of the reflective liquid crystal display panel. Note that FIG. 13 also shows the configuration of the cross section of each of a gate terminal portion and a source terminal portion.
As in the case of the transmissive liquid crystal display panel, the reflective liquid crystal display panel includes the TFT 100, the gate terminal 102, the source terminal 103, the gate terminal pad 104, and the source terminal pad 105. The reflective electrode 107 is used as the pixel electrode, and the reflective electrode 107 is formed on substantially the entire surface of each pixel. As the reflective electrode 107, Ag or Al is used in many cases. In a similar manner as in the transmissive liquid crystal display panel, those components are connected to each other via the contact holes 106.
In any type of the liquid crystal display panels, on the gate terminal 102 and the source terminal 103, each of which serves as the connection portion to be connected with the drive circuit, the gate terminal pad 104 and the source terminal pad 105 are formed. As the gate terminal pad 104 and the source terminal pad 105, a transparent conductive film such as ITO is generally used so as to prevent high resistance due to oxidation of the connection portion caused in a post process, operating environments, and the like. In the transmissive or transflective liquid crystal display panel, the transmissive electrode 101 made of ITO or the like is formed. An ITO film made of indium oxide series or tin oxide series and thus used as the gate terminal pad 104, the source terminal pad 105, or the transmissive electrode 101 has a low coverage characteristic (see Japanese Unexamined Patent Application Publication No. 11-281992). For this reason, as shown in FIGS. 9, 11, and 13, in the contact hole 106 serving as the connection portion between the transmissive electrode 101 and the TFT, or in the contact hole 106 serving as the connection portion between the terminal and the pad, a coverage defect portion 108 of the ITO film is generated due to steps formed at the contact holes. In other words, a disconnection of the transmissive electrode 101, the gate terminal pad 104, and the source terminal pad 105 is likely to occur. As a result, a display defect or display unevenness of the liquid crystal display panel occurs.
Further, in recent years, to reduce a frame size of a liquid crystal display panel, a drive circuit is incorporated into a peripheral portion of the liquid crystal display panel. As a result, contact holes are formed in a peripheral portion of the liquid crystal display panel, in particular, in the vicinity of a sealing material which is formed so as to surround the circumference of the liquid crystal display panel. The contact holes formed in the vicinity of the sealing material also have the ITO film formed in an upper layer of the wirings, electrodes, or terminals that are made of metal or the like. In this case, since the ITO film has a low coverage characteristic, the wirings, electrodes, or terminals formed below the ITO film are affected by water or impurities entering from the sealing material. As a result, the wirings, electrodes, or terminals are corroded or electrically corroded.
In particular, as a result of studies made by the inventors of the present invention, it has been found that the coverage defect of the ITO film highly frequently occurs in a case of employing a process in which the ITO film is formed in an amorphous state and is then crystallized. In general, in a process for patterning the ITO film, wet etching using liquid chemical is often used. In a case of a crystalline ITO film, it is necessary to use strong acid containing aqueous solution with hydrochloric acid and nitride acid, as the liquid chemical to be used for the wet etching. In this case, when metal thin films made of Al, Ag, or Mo coexist as the gate signal lines, the source signal lines, or the reflective electrodes, there is a fear that those metal thin films are corroded and disconnected during a wet etching process for the ITO film.
On the other hand, an amorphous ITO film can be subjected to wet etching using weak acid such as oxalic acid series aqueous solution. For this reason, even when metal thin films made of Al, Ag, or Mo coexist, there is no fear that those metal thin films are corroded and disconnected. Accordingly, the ITO film is first formed in an amorphous state, and is patterned using oxalic acid series aqueous solution. After that, it is desirable to employ a process in which the ITO film be crystallized by heating, for example, whereby the ITO film is finally chemically stabilized.
However, when the ITO film is changed in phase from the amorphous phase to the crystalline phase, a disordered array of atoms is changed into an ordered array of atoms. Along with such a change, a distance between crystal atoms becomes smaller, whereby volume contraction occurs. As a result, the ITO film is applied with a tensile stress from a substrate. Accordingly, a step disconnection of the ITO film occurs more easily particularly in a step portion such as a contact hole. Measurement results regarding a film stress of an ITO film formed above an Si substrate, which is obtained by the inventors of the present invention, are shown in Table 1. The film stress of an amorphous ITO film obtained immediately after deposition is −200 MPa. That is, a compressive stress of 200 MPa is applied. On the other hand, the film stress obtained by heating the ITO film at 300° C. to be crystallized is +200 MPa. That is, a tensile stress of 200 MPa is applied. From the above results, it is apparent that, in the case of forming the amorphous ITO film, the tensile stress is applied with the progress of the crystallization. On the other hand, in the case of forming the crystallized ITO film, a crystal phase of the ITO film is not changed after heat treatment, with the result that the film stress of the ITO film is not changed. As described above, when the contact hole is coated with the amorphous ITO film, excellent etching workability is obtained, while the coatability is poor. As a result, corrosion of metal wirings due to ingress of water or impurities may occur. To solve the problems, for example, wiring in a base layer can be formed into a two-layer structure (For example, see Japanese Unexamined Patent Application Publication Nos. 10-10576 and 2004-205550).
TABLE 1(a) Film stress of ITO film formed in amorphous stateamorphous film obtained immediately after deposition −200 Mpa (compressive stress)polycrystalline film obtained after heat treatment +200 MPa (tensile stress) (b) Film stress of ITO film formed in polycrystalline statepolycrystalline film obtained immediately after deposition +200 MPa (tensile stress)polycrystalline film obtained after heat treatment +200 MPa (tensile stress)
However, merely by laminating a metallic material, it is impossible to prevent wiring in a base layer from being corroded. Further, there is no disclosure on the problem of a contact hole formed in an area which is not covered with an alignment layer and into which water and impurities are more likely to enter. For example, there is no disclosure on a contact hole formed in an element (drive circuit, test circuit, or the like) disposed in the vicinity of a sealing material.